FIG. 19 is a diagram illustrating a configuration of a conventional optical transmission device 91 supporting encrypted communication. In FIG. 19, the optical transmission device 91 comprises a light source 95, an optical information modulating section 94, an encryption section 93, an optical transmission channel 993, a decoding section 98, and an optical intensity detecting section 97. The light source 95, the optical information modulating section 94, and the encryption section 93 constitute an optical transmitting circuit 92. The decoding section 98 and the optical intensity detecting section 97 constitute an optical receiving circuit 96. Note that, in FIG. 19, in order to describe an operation of the optical transmission device 91, an eavesdropper's optical receiving circuit 99 comprising an eavesdropper's optical intensity detecting section 992 and a decryption section 991 is also illustrated.
An operation of the optical transmission device 91 thus configured will be described with reference to FIG. 19. The encryption section 93 in the optical transmitting circuit 92 and the decoding section 98 in the optical receiving circuit 96 previously share a source code Ki as an “encryption key”. The encryption section 93 encrypts an information signal Di to be transmitted, using the source code Ki, and outputs the resultant encrypted signal. The optical information modulating section 94 optically modulates light output from the light source 95 using the encrypted information signal output from the encryption section 93, and outputs the resultant signal to the optical transmission channel 993. The optical intensity detecting section 97 receives the optically modulated signal transmitted through the optical transmission channel 993, detects and converts the intensity-modulated light component into an electrical signal, and outputs the electrical signal. The decoding section 98 decodes the output signal from the optical intensity detecting section 97 using the source code Ki to reproduce the information signal Di.
In conventional optical transmission devices as described above, a predetermined code is shared as a “secret key” between a sender and a receiver. The sender encrypts an information signal by performing a predetermined computation process using the code. After transmission, the receiver decodes the signal by performing substantially a reverse computation process using a similar code, to reproduce the original information signal. Thereby, decoding is not possible for the third party other than authorized receivers having the “secret key”, i.e., it is significantly difficult to achieve eavesdropping, thereby making it possible to achieve data communication with a high level of secrecy.    Patent Document 1: Japanese Laid-Open Patent Publication No. 9-205420